Systems, methods, and devices for reducing optical and electrical crosstalk in photodiodes

ABSTRACT

Devices, systems, and methods are provided for reducing electrical and optical crosstalk in photodiodes. A photodiode may include a first layer with passive material, the passive material having no electric field. The photodiode may include a second layer with an absorbing material, the second layer above the first layer. The photodiode may include a diffused region with a buried p-n junction. The photodiode may include an active region with the buried p-n junction and having an electric field greater than zero. The photodiode may include a plateau structure based on etching through the second layer to the first layer, the etching performed at a distance of fifteen microns or less from the buried p-n junction.

TECHNICAL FIELD

This disclosure generally relates to limiting crosstalk in photodiodedevices.

BACKGROUND

Semiconductors increasingly are being used for various applications.Some applications use multiple semiconductors in close proximity to oneanother. The operation of semiconductors, such as photodiodes, mayresult in electrical and/or optical crosstalk between photodiode pixels.The crosstalk may be a source of noise that may increase as pixel pitchdecreases. Fabrication of photodiodes to reduce crosstalk may beinefficient due to a need for electrical passivation of materials withsignificant electric fields, and due to a risk of noise resulting fromsome material used to facilitate electrical passivation. There is a needfor an efficient design of low-noise photodiodes with small pixel pitch.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates example photodiode structures, in accordance withone or more example embodiments of the present disclosure.

FIG. 1B illustrates example photodiode structures, in accordance withone or more example embodiments of the present disclosure.

FIG. 1C illustrates a portion of photodiode structures, in accordancewith one or more example embodiments of the present disclosure.

FIG. 2 illustrates an example homojunction p-n photodiode, in accordancewith one or more example embodiments of the present disclosure.

FIG. 3 illustrates an example heterojunction p-i-n photodiode, inaccordance with one or more example embodiments of the presentdisclosure.

FIG. 4 illustrates an example heterojunction avalanche photodiode, inaccordance with one or more example embodiments of the presentdisclosure.

FIG. 5 illustrates a graph showing average dark count rate forphotodiode devices, in accordance with one or more example embodimentsof the present disclosure.

FIG. 6A illustrates an example top view of a photodiode array, inaccordance with one or more example embodiments of the presentdisclosure.

FIG. 6B illustrates an example side cross-section view of the photodiodearray of FIG. 6A, in accordance with one or more example embodiments ofthe present disclosure.

FIG. 6C illustrates example avalanching pixels of the photodiode arrayof FIG. 6A, in accordance with one or more example embodiments of thepresent disclosure.

FIG. 7 illustrates an example top view of a photodiode array, inaccordance with one or more example embodiments of the presentdisclosure.

FIG. 8 illustrates an example side cross-section view of the portion ofthe photodiode structures of FIG. 1C, in accordance with one or moreexample embodiments of the present disclosure.

FIG. 9 illustrates a flow diagram for a process for forming aphotodiode, in accordance with one or more example embodiments of thepresent disclosure.

Certain implementations will now be described more fully below withreference to the accompanying drawings, in which various implementationsand/or aspects are shown. However, various aspects may be implemented inmany different forms and should not be construed as limited to theimplementations set forth herein; rather, these implementations areprovided so that this disclosure will be thorough and complete, and willfully convey the scope of the disclosure to those skilled in the art.Like numbers in the figures refer to like elements throughout. Hence, ifa feature is used across several drawings, the number used to identifythe feature in the drawing where the feature first appeared will be usedin later drawings.

DETAILED DESCRIPTION

Example embodiments described herein provide certain systems, methods,and devices for reducing electrical and optical crosstalk inphotodiodes.

Photodiodes are semiconductor devices that may convert light intoelectrical current. Photodiodes may be elements of sensors, and may bearranged in an array in which pixels of the photodiode array may have ap-n junction. A p-n junction may be formed when p-type and n-typematerials are arranged in contact with each other. Current may flow fromone type of material to the other to create a diode.

Buried p-n junctions may be used in photodiodes, such as in avalanchephotodiodes (APDs), which may refer to photodiodes that may generatelarge electrical current signals in response to receiving a low-poweredoptical signal. An APD may be biased by applying a voltage across theAPD, resulting in a significant (e.g., non-zero) electrical field. Freeelectrical carriers may be generated in an absorption layer of the APDand injected into a multiplication region (e.g., avalanche region) ofthe APD. The absorption layer may absorb energy from light to generatefree charge carriers. The multiplication layer may be a region of theAPD in which the free charge carriers multiply to generate detectableelectrical current. The free carriers may accelerate in themultiplication layer, allowing them to create additional free carriers(e.g., a process referred to as “impact ionization”), and the additionalfree carriers also may accelerate (e.g., due to the electric fieldpresence) to create more free carriers. This multiplication of freecarriers may be referred to as avalanche multiplication. When lightenters a photodiode, electron-hole pairs may be generated when theenergy from the light exceeds a band gap energy. An electric field maycause the electrons to drift to the n-type material and the holes (e.g.,electron holes) to drift to the p-type material. The stronger theelectric field, the more drift of the electrons and holes.

An active material of a photodiode may refer to a material having afinite, non-zero electric field. A non-active or passive material mayrefer to a material in which no electric field is present (e.g., anelectric field of zero). The rate at which an electric field decreases(e.g., to zero) from an active region to a passive region may depend onthe doping used. For example, an electrical field of one photodiode maydrop off at a distance closer to a diffused region than in anotherphotodiode because of the doping used in the photodiodes.

The use of buried p-n junctions in a photodiode may facilitateelectrical passivation, but may result in a remaining presence of anabsorber material that may contribute to increased noise associated withcrosstalk effects. A non-zero electric field (e.g., caused by an appliedvoltage across a photodiode) may be present in at least a portion ofabsorber material. For example, the absorber material may at leastpartially surround an active p-n junction. The further away from theactive p-n junction, the weaker the electric field of the absorbermaterial may be.

Some photodiodes use a “mesa” structure to create an active region. Amesa structure on a semiconductor may refer to an area where material ofthe semiconductor has not been removed (e.g., etched away), resulting ina flat-topped surface that rises above a surrounding semiconductorsubstrate. Mesa structures for semiconductors may be formed by etchingaway non-active material and leaving active material for the mesa withnon-planar passivation. To reduce crosstalk, non-active absorbermaterial may be etched away because the non-active material maycontribute to crosstalk between photodiode pixels. The crosstalk may bea source of noise that increases rapidly with decreasing pixel pitch(e.g., the density of pixels in a cluster of photodiode array pixels).Mesa formation, however, may require electrical passivation of surfaceshaving high electric fields, resulting in fabrication challenges andinefficiencies. For example, forming a mesa structure by etching into anactive region where the electric field strength is greater than zero oretching into an inactive region too close to the active region mayrequire passivation of material even though crosstalk may be reduced byremoving some of the material.

Therefore, there is a need to design a low-noise photodiode array withsmall pixel pitch.

In one or more embodiments, the use of a mesa structure may be combinedwith a buried p-n junction to form a photodiode array with reducedcrosstalk and noise. The photodiode array may be formed by removingabsorber material from a device having a buried p-n junction to mitigatecrosstalk while maintaining high-quality passivation. For example, themore material etched away or otherwise removed, the more crosstalk maybe reduced. However, the removal of absorber material further away froman active region may result in the remaining absorber material beingwithin the active region and having a non-zero electric field, or beingin close proximity to material having a non-zero electric field, therebynecessitating passivation. The surface electric field intensities may bereduced and may be tuned by adjusting the distance between the activeabsorber material and the removed absorber material. Removal of absorbermaterial may be achieved using geometric configurations such astrenches, pits, end caps, wells, and troughs as described furtherherein, and may allow for a significant reduction in the dark carriergeneration, which manifests as dark current in a linear-mode APD or asthe dark count rate (e.g. the average rate of counts without incidentlight) of a single-photon avalanche diode. An active region of thephotodiode array may be formed by etching away at least a portion of theabsorber region to form a mesa structure with non-planar passivation,and the active region may be formed by dopant diffusion to create aburied p-n junction with planar passivation. To optimize passivation byavoiding a need to passivate material with a high electric field, atleast some absorber material around the active p-n junction may not beremoved by etching. Etching further away from the p-n junction andleaving more absorber material may result in significant crosstalk, butetching closer to the p-n junction and leaving less (if any) absorbermaterial may result in a need to passivate material having significantelectric field strength. Therefore, the hybrid use of a mesa structureand a buried p-n junction may balance a goal of reducing crosstalk byetching absorber material with a goal of avoiding a need to passivatematerials with a significant electric field.

In one or more embodiments, the electric field strength of thephotodiode array may be tailored based on the amount of absorbermaterial that is left around the buried p-n junction after etching awaysome of the absorber material. For example, rather than etching away allof the non-active absorber material around the active material in aphotodiode array, passivation may be achieved by leaving some absorbermaterial as a buffer between the exposed portions of active material,but not so much absorber material that significant crosstalk results.The optimized amount of etching may answer a question of how far into orhow close to the active region to etch in order to minimize bothpassivation and crosstalk. The distance from the diffused region atwhich absorber material may be etched may be dependent on the electricfield strength at the distance, and based on any need to passivate anymaterial that remains after etching.

In some focal plane arrays, carrier diffusion time constants may haverelaxed requirements. For example, for some InGaAs PIN detectors (e.g.,un-doped detectors) operating at video rates (e.g., 24-60 Hz),collection times for free carriers may be on the order of microsecondsor milliseconds. In such cases, no absorber material needs to be removedfrom a photodiode array, and all of the absorber material in a pixelarea of the photodiode array may contribute to the overall fill factorwhen the pixel dimension is on the order of a minority carrier diffusionlength. In contrast, for applications requiring faster time constants,there may be drawbacks to leaving absorber material within an effectivepixel because the absorber material may have slow time constants forcarrier collection. Light detection and ranging (LIDAR) applications areamong applications that may require fast time constants.Telecommunications also may require fast time constants, and may requirehigh linearity. For example, analogue telecommunications receivers mayexperience significant waveform distortion with slow time constants.

To address the need for fast time constants in some applications, anoptical use may ensure that no photons reach the non-active (i.e.field-free) absorber material of a photodiode array. However, whenphotons are generated within a semiconductor itself, it may not befeasible to ensure that the photons do not reach the absorber material.Such “crosstalk” photons generated during a very large avalanchedetection event may be absorbed directly in a neighboring pixel activeregion (e.g., “optical crosstalk”) or absorbed in a non-active regionwhere the generated electrical carriers may diffuse to a neighboringactive region (e.g., “diffusive crosstalk”). This issue may be a problemfor applications requiring fast time constants. For example, an InGaAsabsorber material may have long carrier diffusion lengths (e.g., 50-200um) and lifetimes (e.g., 1-100 us). Because carriers may be long-livedand may diffuse relatively long distances, the carriers may migrate longdistances outside of the active regions of the photodiode array to becollected and trigger dark counts at hundreds of nanoseconds ormicroseconds later, giving rise to false detection events much laterthan the time corresponding to the actual arrivals of signal photons.

In one or more embodiments, a photodiode array may include pits,trenches, end caps, and/or troughs. For example, an avalanche photodiodearray may include multiple layers of materials, such as a buffer layer,an absorber layer above the buffer layer, active andmultiplication/avalanche regions above the absorber layer, and troughsetched from the active region to the buffer layer. The troughs may allowfor a portion of the top surface of the absorber layer to be exposed andfor a portion of the buffer to be exposed, forming a mesa structure inwhich the active material forms the top of the mesa, with some absorbermaterial left around the active material, and some absorber materialetched away to form the mesa with part of the buffer exposed at a lowerlevel. The photodiode array may have multiple active regions surroundedby absorber material, and some absorber material etched away to exposethe buffer layer and form a mesa structure. The active material mayinclude a buried p-n junction to leverage the electrical passivationfacilitated by the buried p-n junction, and some of the noisy absorbermaterial may be etched away from around the buried p-n junction toreduce crosstalk.

In one or more embodiments, a hybrid design of photodiode arrays thatuses both buried p-n junctions and an etched mesa structure may supportGeiger-mode operations. In Geiger-mode operations, APDs in an array maybe biased above a breakdown voltage to allow a photon to trigger anavalanche.

The above descriptions are for purposes of illustration and are notmeant to be limiting. Numerous other examples, configurations,processes, etc., may exist, some of which are described in greaterdetail below. Example embodiments will now be described with referenceto the accompanying figures.

FIG. 1A illustrates example photodiode structures, in accordance withone or more example embodiments of the present disclosure.

Referring to FIG. 1A, a photodiode structure 100 is showntwo-dimensionally from a top view. The photodiode structure 100 may be aplanar-diffused structure with a non-active material 102 (e.g. materialhaving zero electric field strength) surrounding active material 104(e.g., material having non-zero electric field strength). The non-activematerial 102 may be removed (e.g., etched away) to form a photodiodestructure 106 (e.g., shown from a top view). The photodiode structure106 may include the active material 104 of the photodiode structure 100,along with a buffer layer 108 (e.g., exposed by removing the entirety ofthe non-active material 102 from the photodiode structure 100). Thephotodiode structure 106 therefore may include no non-active absorbermaterial surrounding the active material 104, so the photodiodestructure 106 may require significant passivation of the active material104.

Referring to FIG. 1B, a photodiode structure 150 is showntwo-dimensionally from a top view. The photodiode structure 150 may be aplanar-diffused structure with the non-active material 102 (e.g.material having zero electric field strength) surrounding the activematerial 104 (e.g., material having non-zero electric field strength).Unlike the photodiode structure 106 of FIG. 1A, some of the non-activematerial 102 may remain after etching. The buffer layer 108 may beexposed where portions of the non-active material 102 have been removed.In the photodiode structure 150, the non-active material 102 forms arectangular region around the active material 104. In this manner, theremaining non-active material 102 after etching may result in an areawith zero electric field, thereby avoiding a difficult process ofpassivating material having a high electric field strength.

Still referring to FIG. 1B, a photodiode structure 160 is showntwo-dimensionally from a top view. The photodiode structure 160 may be aplanar-diffused structure with the non-active material 102 (e.g.material having zero electric field strength) surrounding the activematerial 104 (e.g., material having non-zero electric field strength).Unlike the photodiode structure 106 of FIG. 1A, some of the non-activematerial 102 may remain after etching. The buffer layer 108 may beexposed where portions of the non-active material 102 have been removed.In the photodiode structure 160, the non-active material 102 forms acircular region around the active material 104. In this manner, theremaining non-active material 102 after etching may result in an areawith zero electric field, thereby avoiding a difficult process ofpassivating material having a high electric field strength.

Referring to FIG. 1A and FIG. 1B, the removal of at least a portion ofthe non-active material 102 may result in a mesa structure. As shown inFIG. 1B, the active material 104 may be formed by using a buried p-njunction, which may be above the buffer layer 108. The buffer layer 108with a higher layer formed by the active material 104 may result in amesa structure with a p-n junction. The photodiode structure 150 and thephotodiode structure 160 may refer to APDs with an array of photodiodes.

FIG. 1C illustrates a portion 170 of photodiode structures, inaccordance with one or more example embodiments of the presentdisclosure.

Referring to FIG. 1C, the portion 170 may be similar to the photodiodestructure 150. For example, the portion 170 may have electrical traces(e.g., trace 172, trace 174, trace 176) with anode contact points (e.g.,contact point 178 of the trace 172, contact point 180 of the trace 174,contact point 182 of the trace 176). The anode contact points may bepositioned within respective diffused regions (e.g., the contact point178 may be positioned within diffused region 184; the contact point 180may be positioned within the diffused region 186; and the contact point182 may be positioned within diffused region 188). The diffused regionsmay refer to multiplication/avalanche regions where avalanching mayoccur. Plateaus (e.g., plateau 190, plateau 192, plateau 194, plateau195, plateau 196, and plateau 197) may be created by removal (e.g.,etching) of material, resulting in the formation of trenches (e.g.,trench 198) between the plateaus (e.g., the trench 198 may be furtherinto the page than the plateaus).

Where the trench 198 is etched, and therefore where the plateaus areformed relative to the diffused regions, represents a selection of thedistance d1 (e.g., a distance between the diffused region 186 and thetrench 198) to minimize crosstalk and optimize electrical passivation.For example, the smaller the distance d1, the more likely the electricalfield where the trench 198 is formed may be greater than zero, thereforerequiring more significant electrical passivation. The larger thedistance d1, the more likely the electrical field where the trench 198is formed may be zero. In this manner, the distance d1 is a factor inthe electrical field strength of the sides of the plateaus formed by theetching that results in the trench 198.

The cross-section lines are the basis for FIG. 8.

FIG. 2 illustrates an example homojunction p-n photodiode 200, inaccordance with one or more example embodiments of the presentdisclosure.

Referring to FIG. 2, the homojunction p-n photodiode 200 may include ap-diffused region 202 at least partially positioned within an n-absorberlayer 204. The n-absorber layer 204 may be positioned on a buffer layer208. Where the homojunction p-n photodiode 200 is active (e.g., exhibitsan electric field E>0) may depend a distance d1 from the p-diffusedregion 202. As shown by a graph 250, the electric field E of thehomojunction p-n photodiode 200 may be active up until a distance d2,and then inactive beyond the distance d2. In this manner, when then-absorber layer 204 is removed (e.g., etched) at a location within thedistance d1 from the p-diffused region 202, the remaining n-absorberlayer 204 may be active, and therefore may require passivation. When then-absorber layer 204 is removed at a location outside of (e.g., greaterthan) the distance d1 from the p-diffused region 202, the remainingn-absorber layer 204 may be inactive, and therefore may avoid the needfor passivation. However, more of the n-absorber layer 204 remainingafter etching may result in greater noise due to crosstalk effects. Inthis manner, the location at which the n-absorber layer 204 may beremoved may be a balancing act that maximizes the amount of then-absorber layer 204 removed to reduce crosstalk effects whileminimizing passivation that may be necessitated by removing too much ofthe n-absorber layer 204 (e.g., such that the remaining n-absorber layer204 is active). If the n-absorber layer 204 is removed within the activeregion and exposes a surface with a higher electric field intensity, asindicated by the graph 250, then a higher quality passivation layer willbe needed to adequately passivate this exposed active region surface.

FIG. 3 illustrates an example heterojunction p-i-n photodiode 300, inaccordance with one or more example embodiments of the presentdisclosure.

Referring to FIG. 3, the heterojunction p-i-n photodiode 300 may includea p+ diffused region 302 positioned at least partially within a n− caplayer 304 and an n− absorption layer 306 (e.g., which may be positionedbelow the n− cap layer 304), and a n+ buffer layer 308 positioned belowthe n− absorption layer 306. Where the heterojunction p-i-n photodiode300 is active (e.g., exhibits an electric field E>0) may depend adistance d3 and/or the distance d4 from the p+ diffused region 302. Asshown by a graph 350, the electric field E of the heterojunction p-i-nphotodiode 300 may be constant up until the distance d3, and then the Efield may decline from d3 to d4, beyond which the heterojunction p-i-nphotodiode 300 may become inactive. In this manner, when the n− caplayer 304 and the n− absorption layer 306 are removed (e.g., etched) ata location within the distance d3 or d4 from the p+ diffused region 302,the remaining n− cap layer 304 and n− absorption layer 306 may beactive, and therefore may require passivation. When the n− cap layer 304and the n− absorption layer 306 are removed at a location outside of(e.g., greater than) the distance d4 from the p+ diffused region 302,the remaining n− cap layer 304 and n-absorption layer 306 may beinactive, and therefore may avoid the need for passivation. However,more of the n− cap layer 304 and n− absorption layer 306 remaining afteretching may result in greater noise due to crosstalk effects. In thismanner, the location at which the n− cap layer 304 and n− absorptionlayer 306 may be removed may be a balancing act that maximizes theamount of the n− cap layer 304 and n− absorption layer 306 removed toreduce crosstalk effects while minimizing passivation that may benecessitated by removing too much of the n− cap layer 304 and n−absorption layer 306 (e.g., such that the remaining n− cap layer 304 andn− absorption layer 306 are active).

FIG. 4 illustrates an example heterojunction avalanche photodiode (APD)400, in accordance with one or more example embodiments of the presentdisclosure.

Referring to FIG. 4, the heterojunction APD 400 may include a p+diffused region 402 at least partially positioned within a cap layer404, and the cap layer 404 may be positioned above an n− absorptionlayer 406. Below the n− absorption layer 406, the heterojunction APD 400may include an n+ buffer layer 408. Avalanching may occur within the caplayer 404 (e.g., at a portion 410 of the cap layer 404 proximal enoughto the p+ diffused region 402, where the electric field strength may bestrong enough to give rise to avalanche gain). The portion 410 of thecap layer 404 may be referred to as an avalanche region, which mayextend to the n− absorption layer 406.

Where the heterojunction APD 400 is active (e.g., exhibits an electricfield E>0) may depend a distance d5 and/or the distance d6 from the p+diffused region 402. As shown by a graph 450, the electric field E ofthe heterojunction APD 400 may be constant up until the distance d5(e.g., the edge of the avalanche region), and then the E field maydecline from d5 to d6, beyond which the heterojunction APD 400 maybecome inactive. In this manner, when the cap layer 404, the n−absorption layer 406, and the n+ buffer layer 408 are removed (e.g.,etched) at a location within the distance d5 or d6 from the p+ diffusedregion 402, the remaining cap layer 404, n-absorption layer 406, and n+buffer layer 408 may be active, and therefore may require passivation.When the cap layer 404, the n− absorption layer 406, and the n+ bufferlayer 408 are removed at a location outside of (e.g., greater than) thedistance d6 from the p+ diffused region 402, the cap layer 404, n−absorption layer 406, and n+ buffer layer 408 may be inactive, andtherefore may avoid the need for passivation. However, more of the caplayer 404, n− absorption layer 406, and n+ buffer layer 408 remainingafter etching may result in greater noise due to crosstalk effects. Inthis manner, the location at which the cap layer 404, n− absorptionlayer 406, and n+ buffer layer 408 may be removed may be a balancing actthat maximizes the amount of the cap layer 404, n− absorption layer 406,and n+ buffer layer 408 removed to reduce crosstalk effects whileminimizing passivation that may be necessitated by removing too much ofthe cap layer 404, n− absorption layer 406, and n+ buffer layer 408(e.g., such that the cap layer 404, n− absorption layer 406, and n+buffer layer 408 are active). If the n− absorption layer 406 is removedwithin the active region and exposes a surface with a higher electricfield intensity, as indicated by the graph 450, then a higher qualitypassivation layer will be needed to adequately passivate this exposedactive region surface.

FIG. 5 illustrates a graph 500 showing average dark count rate forphotodiode devices, in accordance with one or more example embodimentsof the present disclosure.

Referring to FIG. 5, the dark count rate is shown on the vertical axiswith a metric of hertz (Hz), and overbias is shown on the horizontalaxis with a metric of volts (V). The dark count rate 502 of anun-optimized device (e.g., a device without the hybrid mesa structurewith buried p-n junction and selective removal of non-active material)may experience a significantly higher dark count rate than the darkcount rate 504 experienced by an optimized device (e.g., a device usingthe hybrid mesa structure with buried p-n junction and selective removalof non-active material), such as a device using the photodiode structure150 or the photodiode structure 160 of FIG. 1B. For example, anoptimized device may experience a thirty-time reduction in dark countrate when compared to an un-optimized device. In this manner, the graph500 shows benefits of using enhanced methods and structures to reduceelectrical and optical crosstalk in photodiodes.

FIG. 6A illustrates an example top view of a photodiode array 600, inaccordance with one or more example embodiments of the presentdisclosure.

Referring to FIG. 6A, the photodiode array 600 may include traces 602(e.g., metal traces) and one or more trenches 604 (e.g., indentions),which may be dry-etched or otherwise fabricated. The traces 602 mayinclude conductive material (e.g., copper or otherwise) that allow forthe flow of electricity. The one or more trenches 604 may preventoptical crosstalk as well as diffusive crosstalk. The photodiode array600 may be an APD or other type of photodiode.

FIG. 6B illustrates an example side cross-section view of the photodiodearray 600 of FIG. 6A, in accordance with one or more example embodimentsof the present disclosure.

Referring to FIG. 6B, the photodiode array 600 may include an anode 652(e.g., a charged electrode in one of the traces 602, through which biasvoltage is applied from the outside) at least partially covering aportion a diffused region 654 (e.g., a degenerately doped region wherethe electric field strength is zero except within a proximity, such asaround 100 nanometers, of a p-n junction 655). The diffused region 654may include the p-n junction 655 (e.g., a buried p-n junction). Thediffused region 654 may extend at least partially into a cap layer 658(e.g., an i-InP cap, or another material). Avalanching may occur withinthe cap layer 658 (e.g., at a portion 656 of the cap layer 658 proximalenough to the diffused region 654, where the electric field strength maybe strong enough to give rise to avalanche gain). The portion 656 of thecap layer 658 may be referred to as an avalanche region. Below the caplayer 658 may be an absorber layer 660 (e.g., an I-InGaAS absorbermaterial or another material), and below the absorber layer 660 may be abuffer layer 662 (e.g., an n+−InP material or another material). Anelectrical carrier 664 created by photon absorption in the absorberlayer 660 may diffuse along a path 666. The electrical carrier 664 maydiffuse to the avalanche region 656 (e.g., diffusive crosstalk).

In one or more embodiments, crosstalk can be particularly troubling inapplications with fast time constants. An InGaAs absorber material(e.g., used in the absorber layer 660), for example, may allow for longcarrier diffusion lengths and lifetimes (e.g., diffusion lengths of−50-200 lifetimes from ˜1-100 μs). Because carriers (e.g., theelectrical carrier 664) may be long-lived and may diffuse quite far,carriers may migrate from long distances outside the avalanche region656 to be collected and trigger dark counts at 100 s of nanoseconds oreven microseconds later, giving rise to false detection events muchlater than the time corresponding to true signal photon arrivals.

In one or more embodiments, at least some of the absorber layer 660 maybe removed (e.g., etched away) in order to mitigate diffusive crosstalk.The absorber layer 660 that may be at least partially removed from thephotodiode array 600 may include the one or more trenches 604 of FIG.6A. The optimized amount of the absorber layer 660 removed may answer aquestion of how far into or how close to the active region to etch inorder to both optimize passivation and minimize crosstalk. The distancefrom the avalanche region 656 at which the absorber layer 660 may beetched may be dependent on the electric field strength at the distance,and based on any need to passivate any material that remains afteretching. For example, FIG. 1A represents complete removal of thenon-active portion of the absorber layer 660 (e.g., corresponding to thenon-active material 102), exposing the buffer layer 662 (e.g.,corresponding to the buffer layer 108) below. FIG. 1B represents partialremoval of the non-active portion of the absorber layer 660, leavingsome of the absorber layer 660 (e.g., corresponding to the non-activematerial 102), and exposing some of the buffer layer 662 (e.g.,corresponding to the buffer layer 108) below. The lateral distance fromthe avalanche region 656 to the buffer layer 662 may be based on theelectric field strength at the edges of the unetched material at theclosest location of the exposed buffer layer 662 to the avalanche region656 due to the at least partial removal of the absorber layer 660. Theburied p-n junction 655 may be passivated electrically, but electricfield intensity extending laterally from the p-n junction 655 mayrequire leaving some of the absorber layer 660, which may result incrosstalk as represented by the path 666 of the electrical carrier 664.Therefore, the photodiode array 600 may benefit from removing some ofthe active portion of the absorber layer, but not so much that theexposed surfaces experience an electric field intensity that it toolarge to passivate.

As shown by graph 670, the electric field strength E within theavalanche region is non-zero, and outside the avalanche region may benon-zero, but declining toward zero as the lateral distance from the p-njunction 655 approaches the distance d8.

FIG. 6C illustrates example avalanching pixels of the photodiode array600 of FIG. 6A, in accordance with one or more example embodiments ofthe present disclosure.

Referring to FIG. 6C, the top view of the photodiode array 600 of FIG.6A is shown in more detail with both optical crosstalk and diffusivecrosstalk. Photons (e.g., photon 675, photon 677, photon 679) may beemitted by an avalanche (e.g., of an avalanching pixel 678) may resultin electrical carriers 680 created by photon absorption. The result maybe optical crosstalk at one or more pixels 682 (e.g., carrier generationwithin the active regions of individual pixels). The avalanching pixel678 may represent an initial avalanching pixel that may generateelectrical carriers 688, which may follow a path 681 to induce asubsequent avalanche at pixel 684—a manifestation of diffusive crosstalkin which the pixel 684 may collect the electrical carriers 688). In aregion 690 where photon absorption may create diffusion carriers,diffusive crosstalk may occur as the electrical carriers 688 diffusealong the path 681. In this manner, the avalanche at the pixel 678 mayresult in secondary avalanches at the pixel 684. In one or moreembodiments, to reduce such optical and diffusive crosstalk, thephotodiode array 600 may benefit from removal of at least some of theabsorber layer 660 of FIG. 6B as described above.

In one or more embodiments, the buried p-n junction 655 of FIG. 6B maybe used in the photodiode array 600, such as in APDs. Free electricalcarriers (e.g., the electrical carrier 664) may be generated in theabsorber layer 660 of the APD and injected into the avalanche region 656of the APD. The absorber layer 660 may absorb energy from light togenerate free charge carriers. The avalanche region 656 may be a regionof the APD in which the free charge carriers multiply to generatedetectable electrical current. The free carriers may accelerate in theavalanche region 656, allowing them to create additional free carriers(e.g., a process referred to as “impact ionization”), and the additionalfree carriers also may accelerate (e.g., due to the electric fieldpresence) to create more free carriers. This multiplication of freecarriers may be referred to as avalanche multiplication. When lightenters the photodiode array 600, electron-hole pairs may be generatedwhen the energy from the light exceeds a band gap energy. An electricfield may cause the electrons (e.g., the electrical carrier 664 of FIG.6B) to drift to the substrate, and the holes (e.g., electron holes) todrift to the p-type material of the p-n junction 655. The stronger theelectric field, the more drift of the electrons and holes.

FIG. 7A illustrates an example top view of a photodiode array 700, inaccordance with one or more example embodiments of the presentdisclosure.

Referring to FIG. 7, the photodiode array 700 may be similar to thephotodiode array 600 of FIG. 6A. For example, the photodiode array 700may include one or more trenches 702 (e.g., similar to the one or moretrenches 604 of FIG. 6A), and pits 704 (e.g., removed portions down tothe buffer layer 662 of FIG. 6B), and traces 706. The part of theabsorber layer 660 of FIG. 6B that may be removed from the photodiodearray 600 (or the photodiode array 700) may include any of the one ormore trenches 702, and any of the pits 704.

In one or more embodiments, material around the traces 706 may be etchedaway or otherwise removed to generate the mesa structure as shown inFIG. 8. The etching or other removal of material may generate the one ormore trenches 702 and/or the pits 704.

FIG. 8 illustrates an example side cross-section view of the portion 170of photodiode structures of FIG. 1C, in accordance with one or moreexample embodiments of the present disclosure.

Referring to FIG. 8, the portion 170 represented by the cross-sectionview may include the contact point 182 of FIG. 1C, at least partiallydisposed over a portion of the diffused region 188. The diffused region188 may include a p-n junction 805 (e.g., a buried p-n junction such asthe p-n junction 655 of FIG. 6B). Below the diffused region 188 may bean avalanche region 806 (e.g., similar to the avalanche region 656 ofFIG. 6B). The diffused region 188 may at least partially extend into acap layer 808 (e.g., similar to the cap layer 658 of FIG. 6B). Below thecap layer 808 may be an absorber layer 810 (e.g., similar to theabsorber layer 660 of FIG. 6B), and below the absorber layer 810 may bea buffer layer 812 (e.g., similar to the buffer layer 662 of FIG. 6B).The trench 198 may extend (e.g., via etching) to the buffer layer 812(e.g., through the cap layer 808 and through the absorber layer 810).The trench 198 may be formed by etching away at least some of the caplayer 808 and the absorber layer 810. The distance d1 from the avalancheregion 806 at which the trench 198 is etched may be determined by thepresence of the electrical field at the distance d1. The greater thedistance d1, the less electrical passivation may be required for anyremaining material between the avalanche region 806 and the trench 198,and the easier the fabrication of the portion 170 may be as a result,but the more noise from crosstalk may be present (e.g., as explainedwith respect to FIG. 6C). The smaller the distance d1, the greater thelikelihood of a non-zero electrical field presence that may requireelectrical passivation, resulting in a more difficult fabrication. Inthis manner, the distance d1 may be selected to reduce crosstalk whilealso minimizing the need for electrical passivation.

In one or more embodiments, the buried p-n junction 805 may be used in aphotodiode array, such as in APDs. Free electrical carriers (e.g., theelectrical carrier 664 of FIG. 6B) may be generated in the absorberlayer 810 of the APD and injected into the avalanche region 806 of theAPD. The absorber layer 810 may absorb energy from light to generatefree charge carriers. The avalanche region 806 may be a region of theAPD in which the free charge carriers multiply to generate detectableelectrical current. The free carriers may accelerate in the avalancheregion 806, allowing them to create additional free carriers (e.g., aprocess referred to as “impact ionization”), and the additional freecarriers also may accelerate (e.g., due to the electric field presence)to create more free carriers. This multiplication of free carriers maybe referred to as avalanche multiplication. When light enters theportion 170, electron-hole pairs may be generated when the energy fromthe light exceeds a band gap energy. An electric field may cause theelectrons (e.g., the electrical carrier 664 of FIG. 6B) to drift to thebuffer layer 812, and the holes (e.g., electron holes) to drift to thep-type material of the p-n junction 805. The stronger the electricfield, the more drift of the electrons and holes.

Referring to FIG. 6B and FIG. 8, the diffused region 654 and thediffused region 188 may form a mesa structure with the buried p-njunction 655 and the buried p-n junction 805, respectively (e.g., theplateau 194 of FIG. 1C). The removal of the absorber layer 660 and theabsorber layer 810 may mitigate crosstalk while maintaining high-qualitypassivation. For example, the more material etched away, the morecrosstalk may be reduced, but the removal of absorber material mayresult in the remaining absorber material having a non-zero electricfield or being in close proximity to remaining absorber material havinga non-zero electric field, thereby necessitating passivation. Thesurface electric field intensities may be reduced and may be tuned byadjusting the distance between the active absorber material and theremoved absorber material. Removal of non-active material (e.g., the caplayer 658, the absorber layer 660, the cap layer 808, the absorber layer810) may be achieved using geometric configurations such as trenches,pits, end caps, wells, and troughs (e.g., the trench 198), and may allowfor a significant reduction in the dark count rate (e.g., the averagerate of counts without incident light) when compared to some existingphotodiodes. The diffused region 654 and the diffused region 188 may beformed by etching away at least a portion of the surrounding material(s)to form a mesa structure with non-planar passivation, and the diffusedregion 654 and the diffused region 188 may be formed by dopant diffusionto create the buried p-n junction 655 and the buried p-n junction 805,respectively, with planar passivation. To optimize passivation byavoiding a need to passivate material with a high electric field, atleast some non-active material around the active p-n junction may beremoved by etching. Etching further away from the p-n junction andleaving more absorber material may result in significant crosstalk, butetching closer to the p-n junction and leaving less (if any) absorbermaterial may result in a need to passivate material having significantelectric field strength (e.g., as shown in FIGS. 2-4). Therefore, thehybrid use of a mesa structure and a buried p-n junction may balance agoal of reducing crosstalk by etching non-active material with a goal ofavoiding a need to passivate materials with a significant electricfield.

FIG. 9 illustrates a flow diagram for a process 900 for forming aphotodiode, in accordance with one or more example embodiments of thepresent disclosure.

At block 902, a first layer for a photodiode may be formed, the firstlayer having passive material with no electrical field. For example, thefirst layer may include a buffer layer (e.g., the buffer layer 108 ofFIG. 1A, the buffer layer 108 of FIG. 1B, the buffer layer 662 of FIG.6B, the buffer layer 812 of FIG. 8). The first layer may include ann+−InP material or another material on which one or more additionallayers may be arranged.

At block 904, a second layer for the photodiode may be formed (e.g., thenon-active material 102 of FIG. 1A and FIG. 1B) above the first layer,the second layer having absorber material. The second layer may includea non-active absorber layer (e.g., the absorber layer 660 of FIG. 6B,the absorber layer 810 of FIG. 8). When the photodiode is a homojunctionp-n photodiode (e.g., the homojunction p-n photodiode 200 of FIG. 2),the second layer may include a non-active absorber layer (e.g., then-absorber layer 204 of FIG. 2). When the photodiode is a heterojunctionp-i-n photodiode (e.g., the heterojunction p-i-n photodiode 300 of FIG.3), the second layer may include an n− absorption layer (e.g., the n−absorption layer 306 of FIG. 3). When the photodiode is a heterojunctionAPD (e.g., the heterojunction APD 400 of FIG. 4), the second layer mayinclude an n− absorption layer (e.g., the n− absorption layer 406 ofFIG. 4). An electric field strength in the second layer may depend on adistance from a location of the second layer to an active region formedat block 906.

At block 906, a diffused region having a buried p-n junction for thephotodiode may be formed. The diffused region may be in the form of amesa in which a portion of the diffused region is above at least aportion of at least one of the first layer or the second layer. Thediffused region may include an active region and the second layer. Forexample, referring to FIG. 2, the p-diffused region 202 may be at leastpartially positioned within the n-absorber layer 204. Referring to FIG.3, the p+ diffused region 302 may be positioned at least partiallywithin the n⁻ cap layer 304 and then absorption layer 306. Referring toFIG. 4, the p+ diffused region 402 may be at least partially positionedwithin the cap layer 404, and the cap layer 404 may be positioned abovethe n⁻ absorption layer 406. Referring to FIG. 6B, the diffused region654 may include the buried p-n junction 655 within the cap layer 658,above the absorber layer 660. Referring to FIG. 8, the diffused region188 may include the buried p-n junction 805 within the cap layer 808,above the absorber layer 810. Within the diffused region, the electricfield strength may be non-zero. The electric field strength of thesecond layer may be zero or non-zero depending on the distance from thediffused region to the second layer (see FIGS. 2-4 and the respectiveelectric field graphs, for example).

At block 908, an active region for the photodiode may be determined. Theactive region may refer to a region in which the electrical fieldstrength is non-zero, and the active region may include the buried p-njunction and may include surrounding material where the electric fieldis present.

At block 910, the photodiode may be etched through the second layer tothe first layer (e.g., the trench 198 of FIG. 1 and FIG. 8). Thedistance (e.g., the distance d1 of FIG. 8) from the diffused region atwhich the photodiode is etched to remove at least some of the secondlayer may be determined in a manner that reduces crosstalk and the needfor electrical passivation. For example, rather than etching away all ofthe non-active second layer around the diffused region, passivation maybe achieved by leaving some of the second layer, but not so much of thesecond layer that significant crosstalk results. The optimized amount ofetching may answer a question of how far into or how close to thediffused region to etch in order to minimize both passivation andcrosstalk. The distance from the diffused region at which the secondlayer may be etched may be dependent on the electric field strength atthe distance, and based on any need to passivate any material thatremains after etching.

At block 912, the etching of the second layer may form a plateau (ormesa) structure in which at least a portion of the diffused region is atleast partially above at least a portion of the first layer and/or thesecond layer. As shown in FIG. 8, etching the trench 198 may result inat least a portion of the absorber layer 810 and the buffer layer 812below the diffused region 188. The top views of FIG. 1A and FIG. 1B showthe active material 104 above the buffer layer 108 (e.g., the activematerial 104 coming out of the page more than the buffer layer 108) inthe form of a plateau or mesa. The plateau or mesa formed by etching maybe a result of removed non-active material that, when not removed, maycontribute to electrical and optical crosstalk in the photodiode. Theplateau or mesa may be formed by removing enough material around thediffused region to reduce crosstalk, and may allow for some materialaround the diffused region to remain as long as the electrical fieldstrength within the remaining material around the diffused region isless than a threshold electrical field strength so as to avoid difficultelectrical passivation of the remaining material around the diffusedregion.

The descriptions of the figures are not meant to be limiting.

In one or more embodiments, a method for forming a photodiode mayinclude: forming a first layer comprising passive material, the passivematerial having no electric field; forming a second layer comprising anabsorbing material, the second layer above the first layer; forming adiffused region comprising a buried p-n junction; determining an activeregion comprising the buried p-n junction and having an electric fieldgreater than zero; etching through the second layer to the first layer,the etching performed at a distance of fifteen microns or less from theburied p-n junction (or another distance); and forming a plateaustructure based on the etching. At the distance, an electric field ofthe second layer is zero or non-zero. The photodiode may be an APD. Thedistance may be determined based on a passivation associated with thephotodiode and/or based on the electric field of the active region. Themethod may include forming a third layer above the second layer, whereinthe buried p-n junction is disposed in the second layer and in the thirdlayer. The etching may be from the third layer to the first layer. Themethod may include forming a third layer above the second layer, whereinthe buried p-n junction is disposed in the third layer and is above thesecond layer.

In one or more embodiments, a photodiode may include a first layer withpassive material, the passive material having no electric field. Thephotodiode may include a second layer with an absorbing material, thesecond layer above the first layer. The photodiode may include adiffused region with a buried p-n junction. The photodiode may includean active region with the buried p-n junction and having an electricfield greater than zero. The photodiode may include a plateau structurebased on etching through the second layer to the first layer, theetching performed at a distance of fifteen microns or less from theburied p-n junction. At the distance, an electric field of the secondlayer is zero or non-zero. The photodiode may be an APD. The distancemay be determined based on a passivation associated with the photodiodeand/or based on the electric field of the active region. The photodiodemay include a third layer above the second layer, wherein the buried p-njunction is disposed in the second layer and in the third layer. Theetching may be from the third layer to the first layer. The photodiodemay include a third layer above the second layer, wherein the buried p-njunction is disposed in the third layer and is above the second layer.

As used herein, unless otherwise specified, the use of the ordinaladjectives “first,” “second,” “third,” etc., to describe a commonobject, merely indicates that different instances of like objects arebeing referred to and are not intended to imply that the objects sodescribed must be in a given sequence, either temporally, spatially, inranking, or in any other manner.

It is understood that the above descriptions are for purposes ofillustration and are not meant to be limiting.

Although specific embodiments of the disclosure have been described, oneof ordinary skill in the art will recognize that numerous othermodifications and alternative embodiments are within the scope of thedisclosure. For example, any of the functionality and/or processingcapabilities described with respect to a particular device or componentmay be performed by any other device or component. Further, whilevarious illustrative implementations and architectures have beendescribed in accordance with embodiments of the disclosure, one ofordinary skill in the art will appreciate that numerous othermodifications to the illustrative implementations and architecturesdescribed herein are also within the scope of this disclosure.

Although embodiments have been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the disclosure is not necessarily limited to the specific featuresor acts described. Rather, the specific features and acts are disclosedas illustrative forms of implementing the embodiments. Conditionallanguage, such as, among others, “can,” “could,” “might,” or “may,”unless specifically stated otherwise, or otherwise understood within thecontext as used, is generally intended to convey that certainembodiments could include, while other embodiments do not include,certain features, elements, and/or steps. Thus, such conditionallanguage is not generally intended to imply that features, elements,and/or steps are in any way required for one or more embodiments or thatone or more embodiments necessarily include logic for deciding, with orwithout user input or prompting, whether these features, elements,and/or steps are included or are to be performed in any particularembodiment.

What is claimed is:
 1. An avalanche photodiode, comprising: a firstlayer comprising passive material, the passive material having noelectric field; a second layer comprising an absorbing material, thesecond layer on the first layer; an active region comprising a buriedp-n junction and at least a first portion of the second layer, and theactive region having an electric field greater than zero; and a plateaustructure based on an absence of a second portion of the second layer,the absence of the second portion of the second layer being a distancegreater than zero from the buried p-n junction.
 2. The avalanchephotodiode of claim 1, further comprising a third layer on the secondlayer, wherein the buried p-n junction is disposed in the third layer.3. The avalanche photodiode of claim 2, wherein the absence of thesecond portion of the second layer is based on a removal of the secondportion of the second layer at the distance, and wherein the removal isbased on etching from the third layer to the first layer.
 4. Theavalanche photodiode of claim 1, wherein the absence of the secondportion of the second layer is based on a removal of the second portionof the second layer at the distance, the distance being fifteen micronsor less from the buried p-n junction.
 5. The avalanche photodiode ofclaim 1, wherein at the distance, the electric field in the second layeris zero.
 6. The avalanche photodiode of claim 1 wherein at the distance,the electric field in the second layer is not zero.
 7. The avalanchephotodiode of claim 1 wherein the distance is based on an amount ofpassivation associated with the avalanche photodiode.
 8. The avalanchephotodiode of claim 1, wherein the distance is based on a strength ofthe electric field of the active region.
 9. The avalanche photodiode ofclaim 1, further comprising a third layer on the second layer, whereinthe buried p-n junction is disposed in the second layer and in the thirdlayer.
 10. The avalanche photodiode of claim 9, wherein the absence ofthe second portion of the second layer is based on a removal of thesecond portion of the second layer at the distance, and wherein theremoval is based on etching from the third layer to the first layer. 11.An avalanche photodiode, comprising: a first layer comprising passivematerial, the passive material having no electric field; a second layercomprising an absorbing material, the second layer on the first layer; athird layer on the second layer; an active region comprising a buriedp-n junction and having an electric field greater than zero, the activeregion comprising at least a first portion of the second layer and atleast a first portion of the third layer; and a plateau structure basedon a first absence of a second portion of the second layer and a secondabsence of a second portion of the third layer, the first absence andthe second absence at a distance greater than zero from the buried p-njunction.
 12. The avalanche photodiode of claim 11, wherein the firstabsence and the second absence are based on a removal of the secondportion of the second layer and the second portion of the third layer atthe distance, the distance being fifteen microns or less from the buriedp-n junction.
 13. The avalanche photodiode of claim 11, wherein at thedistance, the electric field in the second layer is zero.
 14. Theavalanche photodiode of claim 11, wherein at the distance, the electricfield in the second layer is not zero.
 15. The avalanche photodiode ofclaim 11 wherein the distance is based on an amount of passivationassociated with the avalanche photodiode.
 16. The avalanche photodiodeof claim 11, wherein the distance is based on a strength of the electricfield of the active region.
 17. The avalanche photodiode of claim 11,wherein the buried p-n junction is disposed in the second layer and inthe third layer.
 18. A method for forming a photodiode, comprising:forming a first layer comprising passive material, the passive materialhaving no electric field; forming a second layer comprising an absorbingmaterial, the second layer on the first layer; forming a diffused regioncomprising a buried p-n junction; forming an active region comprisingthe buried p-n junction and having an electric field greater than zero;and forming a plateau structure based on removal of a portion of thesecond layer to the first layer, the removal performed at a distancefrom the buried p-n junction.
 19. The method of claim 18, wherein at thedistance, the electric field in the second layer is zero.
 20. The methodof claim 18, wherein at the distance, the electric field in the secondlayer is not zero.